This invention is related generally to plasma torches which are generally used for cutting, welding and spray bonding and more specifically to an operation interlock device for such torches.
Plasma torches, also known as electric arc torches, are commonly used for cutting, welding and spray bonding of workpieces and operate by directing a plasma consisting of ionized gas particles toward the workpiece. In the operation of a typical plasma torch, such as illustrated in U.S. Pat. Nos. 4,324,971, 4,170,727 and 3,813,510 assigned to the same assignee as the present invention, a gas to be ionized is supplied to the front end of the torch in front of a negatively-charged electrode. The welding tip which is adjacent to the end of the electrode, at the front end of the torch, has a sufficiently high voltage applied thereto to cause a spark to jump between the electrode and the welding tip thereby heating the gas and causing it to ionize. A pilot DC voltage between the electrode and the welding tip maintains an arc known as the pilot, or non-transferred arc. The ionized gas in the gap appears as a flame and extends externally off the tip where it can be seen by the operator. As the torch head or front end is brought down towards the workpiece, the arc jumps from the electrode to the workpiece since the impedance of the workpiece current path is lower than the impedance of the welding tip current path.
In recent years, touch start plasma torches have been developed such as the Thermal Arc Pak 3 cutting system developed by the assignee. Instead of using a pilot arc to start the torch, as described above, the touch start plasma torch initiates the transferred arc between the cathode and the workpiece by touching the workpiece with the torch tip. No pilot arc will be visible when power to the touch start torch is on. Since there would be no visible flame at the torch head when power is on, torch operators or other persons may accidentally touch the torch head and may get an electrical shock. It is therefore desirable to provide a means to minimize getting electrical shocks by touching the torch head.
Operation interlocks have been used in the past in metal working equipment. For example, an operation interlock device has been employed in pressing machines for pressing threaded inserts into sheet metal. The interlock device prevents large pincer arms from closing to crush an object such as a finger by sensing the electrical resistance of the object to be pressed.